It is possible for Worldgorger Dragon to leave the battlefield before its “enters the battlefield” trigger resolves. If this happens, then the “leaves the battlefield” trigger will have nothing to return. Then its “enters the battlefield” trigger resolves, and all other permanents you control are exiled forever.

6/8/2016

If an Aura is exiled with Worldgorger Dragon, the player who controls the Aura when it enters the battlefield chooses what it will enchant. It doesn’t have to go back to the same place. It can’t enter the battlefield enchanting a permanent that enters the battlefield at the same time.

6/8/2016

If Animate Dead is put onto the battlefield enchanting Worldgorger Dragon, Worldgorger Dragon’s enters-the-battlefield trigger will exile Animate Dead, which causes you to sacrifice Worldgorger Dragon, which causes Animate Dead to return to the battlefield attached to a creature card in a graveyard of your choice. If you can choose a creature card other than Worldgorger Dragon, you must do so after as many iterations of this loop as you’d like. If you can’t choose a different card, and no player chooses to break the loop, the game ends in a draw.